Zendan-e Soleiman

Only 3 km west of the Tacht-e Suleiman complex was originally another geothermal spring, which also formed a sinter cone over the millennia, reaching a height of 110 metres.
At the time of the Sassanids, numerous buildings were erected on the slope of the cone. Experts suspect that this may also have been a sanctuary. 
Later, probably when the spring dried up and left behind a 100 m deep and 70 m wide crater, the place was abandoned. Unfortunately, not much of the buildings and the surrounding wall has survived today. 
But also the "empty" rock cone is a very impressive natural spectacle; not to mention the view to the surrounding landscape.

According to legend, the prophet Solomon was imprisoned inside the rock cone for a while, from where the place got his name (Zendan-e Soleiman).